This invention relates to coating processes and apparatus, and in particular to processes and apparatus for coating surfaces of strip articles of indeterminate length.
In coating operations as herein contemplated, an initially wet, flowable coating material is applied to a surface of a substrate that is at least substantially impervious thereto, for covering the substrate surface with a continuous adherent coating layer. One especially important application of the invention, to which detailed reference will be made for purposes of illustration, is the coating of metal strip with a protective and/or decorative layer of paint or the like, prior to cutting or forming of the strip into shingles, siding or soffit panels, building trim members, or other products.
Metal strip (i.e. strips of sheet metal of indeterminate length, usually stored as coils) is continuously coated, in commercial practice, by advancing the strip longitudinally past a locality where a wet coating material such as paint is applied to one or both major surfaces of the strip, and then through a zone where the coating is cured or dried with heat. Known techniques for applying wet coating material to a strip surface include spraying, transfer from rolls, and deposit of the coating material on the strip surface immediately ahead of a doctor blade or dam which has the purpose of establishing a desired coating thickness. In the latter instance, the blade or dam, as will be understood, has a thin edge extending transversely across and very slightly spaced from the surface to be coated; the deposited wet coating material puddles on the upstream side of the blade or dam and is carried thereunder in a thin layer on the moving surface.
While coating operations using a blade or dam are advantageous from the standpoint of mechanical simplicity, they (like other coating techniques, e.g. spray and roll-coating) do not afford assured or easily attainable high uniformity of coating thickness, especially in the coating of metal strip which commonly has wavy edges, an "oil-canned" central area, or other slight deformations tending to cause variation in the effective spacing between the blade or dam edge and the strip surface and consequently in the thickness of the coating layer determined by that spacing. In order to achieve an adequate coating thickness at all points on the strip surface, therefore, it is commonly necessary to apply a coating layer having a greater average thickness (and thus to consume more coating material) than would be required if the thickness could be made more uniform. This consumption of excess coating material is economically undesirable.
An additional disadvantage of such conventional coating arrangements is the waste of coating material that occurs, e.g. through overflow, owing inter alia to shortcomings in the effectiveness of the metering action provided by these arrangements. Moreover, there is a tendency for air to be picked up in the coating material ahead of the doctor blade, and to become entrapped in the coating, especially at fast coating speeds.
Further complications are encountered when it is attempted by conventional means to provide a coating layer having a striped, streaked, marbleized or otherwise variegated pattern. It has been proposed (in U.S. Pat. No. 3,106,480) to supply paint of different colors to different locations along a common reservoir defined in the nip between two rolls, one of which transfers the paint from the reservoir to a sheet surface to be coated; but in use of blade or dam-type coating arrangements (which, as noted, offer the important advantage of mechanical simplicity) it has heretofore been considered necessary to provide separators for isolating the different colors in the coating material pool or puddle upstream of the dam, as shown for example in U.S. Pat. Nos. 2,695,005 and 3,886,898. Such separators add to the structural complexity of the coating apparatus and prevent or at least greatly restrict the provision of controlled variation in the color patterns produced.